(1) Field of the Invention
This invention relates to dephosphorization and desulfurization methods for a molten iron alloy containing chromium.
(2) Description of the Prior Art
Generally, phosphorus (P) contained in high-chromium steel or stainless steel is well-known as the harmful impurity having an adverse effect on the mechanical property and causing stress-corrosion cracking of steel. However, the dephosphorization for such molten iron alloy containing chromium, even when applied with a usual method for an iron alloy: dephosphorization method of high oxygen potential using CaO--FeO flux or that by oxygen blowing with burned lime (calcium oxide) has been considered to be very difficult because chromium is merely preferentially oxidized in a large quantity and slag is solidified to make hard the progress of oxidization of P.
Conventionally, as the dephosphorization methods for the molten iron alloy containing chromium, the following two methods have been adopted:
1 Dephosphorization by using the Ca--CaF.sub.2 flux in the electro slag and remelting process, and PA0 2 Dephosphorization by using CaC.sub.2 --CaF.sub.2 flux in the ladle.
Both the methods perform the dephosphorization by Ca, in which the dephosphorization proceeds in reducing condition which is given by EQU 3(Ca)+2P.fwdarw.(Ca.sub.3 P.sub.2),
and the latter dephosphorization utilizes Ca produced by the decomposition reaction of CaC.sub.2 given by EQU CaC.sub.2 .fwdarw.Ca+2C.
By either method, however, Ca.sub.3 P.sub.2 is contained in the slag after the dephosphorization treatment. The Ca.sub.3 P.sub.2, as in slag reacts with H.sub.2 O in air forming phosphine (PH.sub.3), which is given by EQU Ca.sub.3 P.sub.2 +3H.sub.2 O.fwdarw.3CaO+2PH.sub.3,
the phosphine is poisonous and having a bad smell of garlic, thereby the problem of the treatment of slag after dephosphorization has been considered to be difficult to solve.